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New Scientist
Physics and Math
Space
News
Earth raises a plasma shield
to battle solar storms
19:00 06 March 2014 by
Lisa Grossman
For similar stories, visit the
Solar System
Topic Guide
Earth can raise shields to protect itself against solar storms. For the first
time, satellites and ground-based detectors have watched as the planet sends
out a tendril of plasma to fight
off blasts of charged solar matter. The discovery confirms a long-standing
theory about Earth's
magnetic surroundings and offers us a way
to keep track of the planet's defences.
"It's changed our thinking about how the system operates," says Joe Borovsky at
the Space Science Institute in Boulder, Colorado, who was
not involved in the research. "Earth doesn't just sit there and take
whatever the solar wind gives it,
it can actually fight back."
Earth is always surrounded by a bubble of magnetism called the magnetosphere,
which protects us from
the bulk of the solar wind, a stream of high-energy
particles constantly flowing from the sun.
But sometimes, the sun's magnetic field lines can directly link up with Earth's
in a process called magnetic reconnection, which opens up cracks in the
magnetosphere. Charged particles
can flow along these lines into Earth's atmosphere, leading to dazzling auroras
as well
as geomagnetic storms that can wreak havoc
on navigation systems and power grids.
Shields up!
Gas in Earth's upper atmosphere is ionised by ultraviolet light from the sun,
and the resulting plasma becomes trapped by magnetic fields in
a doughnut-shaped ring around the planet. Previous observations of this
plasmasphere showed that plumes sometimes
emerge from this region.
Theory had suggested that an extra-strong electric field from the sun can rip
plasma away from the plasmasphere during reconnection, triggering a plume. If
this plume reaches the boundary between
the earthly and solar magnetic fields, it would create a buffer zone of dense
material. This
would make it harder for magnetic field lines
to meet up and spark further reconnection.
But while ground-based measurements can see a plume forming, their resolution
isn't
good enough to tell for sure whether the
material reaches the magnetic boundary.
Brian Walsh at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and
his colleagues have now clinched it. In January 2013, GPS sensors on the ground
mapped electrons
in the upper atmosphere and saw a tendril of increased electron density curling
away from
the north pole, indicating that a plume of
plasma was veering off towards the sun.
Ground truth
At the same time, three of NASA's THEMIS spacecraft, which are designed to
study solar storms, crossed through the magnetic boundary during the
event. The craft saw a 100-fold increase in the number of electrons at
the boundary, which would probably
have been deposited by the plume.
"For the first time, we were able to monitor the entire cycle of this plasma
stretching from the atmosphere to the boundary between Earth's
magnetic field and the sun's," says Walsh. "It gets to that boundary and
helps protect us, keeps these solar
storms from slamming into us."
Not every solar storm generates a plasma plume, which means ground-based
observations will continue to be vital
for understanding the phenomenon.
"To measure things with spacecraft we have to have them in just the right
place, but the ground stations can measure this stuff almost constantly,
" says Walsh. "We want to know, when does the Earth decide to protect
us? By validating this tool, we're
now able to figure that out."
Journal reference: Science,
DOI: 10.1126/science.1247212
If you would like to reuse any content
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and graphics we own the copyright to.
Prepare for the solar storm (Image: NASA)
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